Jacek Piskozub Institute of Oceanology PAS Sopot, Poland

Lecture 4: Air sea interaction in the global scale: from multidecadal variability to Arctic Oscillation

Ho Chi Minh City, December 2007

Jacek Piskozub
Hi Chi Minh City lectures, December 2007

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Ecosystem approach to valuation of marine coasts: examples from Baltic Sea (authored by J. M. Węsławski) Marine aerosol source function: approaching the consensus Ocean as the sink and source of climatically important gases Air sea interaction in the global scale: from multidecadal variability to Arctic Oscillation Climate change threats, Part I: Changes in the climate of the tropic Climate change threats, Part II: Arctic climate and global sea level

What actually is air-sea interaction?

At the ocean-atmosphere interface, ocean and atmosphere exchange fluxes of: Heat: The important heat terms at the surface are the sensible heat flux, the latent heat flux, the incoming solar radiation and the balance of long-wave (infra red) radiation.
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Momentum: The atmosphere imposes a significant wind stress on its surface, and this forces large-scale currents in the ocean.
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Moisture: The ocean can gain moisture from rainfall, or lose it through evaporation.
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Wind shear provides power for most of the ocean surface circulation by means of Ekman transport. The currents result from the balance between gravitational forces and the Coriolis effect (geostrophy) creating the characteristic midlatitude gyres and the circumantarctic circulation.

2. Thermohaline circulation
The thermohaline circulation is that part of the ocean circulation which is driven by fluxes of heat and freshwater across the sea surface and subsequent interior mixing of heat and salt. Stefan Rahmstorf, 2006

Thermohaline circulation (THC) is an effective mean of heat transport from the tropics to the North Atlantic. It is also the source of ocean deep waters, providing ventilation of the World Ocean. THC is the main mechanism of gas (including CO2) exchange between surface waters and ocean deeps.

3. Tides

Amplitude of the M2 tidal constituent (in centimetres) derived from the FES99 model. Cotidal lines indicating the phase every 30 degrees originate at amphidromic points where the tidal range is zero. (Legos/CNRS)

The combined attraction of the Moon and the Sun generates tides on Earth. The M2, is due to the attraction of a 'virtual' Moon placed on a perfectly circular orbit in the Earth's equatorial plane. It has two high and two low tides per day (semi-diurnal wave). The K1 wave, with a diurnal period, reflects declination variations of the Moon and Sun.
http://www.jason.oceanobs.com/html/applications/marees/200010_uk.html

Why dense water return to the surface at all?

Dense (salty and cold) waters can return to the surface only after being mixed with lighter waters. One of the mechanisms (another is wind driven upwelling) is creation of internal waves (green) on ridges an continent slopes by tidal circulation. Internal waves create in turn turbulent mixing (red). This mechanism probably provides most of the 2 TW of mixing needed for THC closure.
Garrett 2003 (Science)‫‏‬

Can we imagine an ocean without air-sea interaction?
Yes, we can. It is possible that Europa, a moon of Jupiter may have a deep water ocean under a permanent thick ice cover. This is almost an ideal model of ocean with no airsea interaction.

What would it circulation look like?
Europa may have a 100 km deep water ocean covered by 30 m of ice. (NASA)

With no air-sea interaction (and almost no air) and no thermohaline circulation, the most we can expect is tide and geothermal convection mixing. The tides would be weak as Europa has a synchronous (the same side always faces Jupiter) rotation period of 3.5 days.

How can we explain any multiannual climate oscillations? The absorbed

amount of heat (in 1022 J) by various components of Earth climate machine: ocean absorbed 84%, land 4% and atmosphere 5%. The missing 7% is the latent heat of ice-sheets and sea ice. The actual heat content share of the ocean even bigger share of the pie as ocean has a long relaxation time for heat fluxes (hundreds of years) and has not yet adjusted to the present greenhouse forcing. Levitus, Antono, Boyer 2005 (Geophysical Res. Letters)‫‏‬

There are many known or suspected multiannual climate oscillations (examples will be shown soon) but it is not possible to explain any of them without air-sea interaction because the heat content of land and atmosphere is too small for a multiannual climate “memory” (and continental moisture can barely explain biannual variability).

What are the possible mechanisms of interannual climate oscillations?
If we observe climate oscillations with a period of multiple years we can suspect either a forced (driven) oscillator, with a periodic forcing controlling its period, or a stochastically forced oscillator (possibly bistable) with a period close to its resonant period. In any case it is very hard to force oscillations when the resonance period is longer than the forcing period (low gain).

Frequency response of an ideal harmonic oscillator. Real climate system are neither harmonic (nonlinearities) nor ideal (they have damping) but the main conclusions are still true. en.wikipedia.org

What need we to do?

To understand any interannual climate oscillation we need to identify either the person pushing the swing (external periodical forcing) or the way someone on the swing can keep it in motion (internal physics of the phenomenon).

Jean-Honore‫‏‬Fragonard:“The‫‏‬Swing”

Two phases of ENSO

La Niña (top) and El Niño (bottom)

El Niño-Southern Oscillation (ENSO): a quasi bistable oceanatmospheric system of 3 to 7 year period with stochastic forcing?

Atlantic Multidecadal Oscillation

Atlantic Multidecadal oscillation is a periodical (60-70 years) warming and cooling of North Atlantic. The temperature anomaly of North Atlantic (top) is used as the AMO index.
Sutton, Hodson 2005 (Science)‫‏‬

Solar forcing: sun activity spectrum from tree ring C 14

Solar activity from tree ring 14C shows many cycles but the shorter important one is 88 year long. It is difficult to explain any multiannual or multidecadal variability of a shorter period by sun only. We also know of no other external decadal periodical forcings.

General circulation models (here HadCM3 model) show that stopping MOC would create a characteristic climate pattern of northern cooling (especially in North Atlantic) and southern warming (more diffuse). Strengthening MOC has an inverse effect.
Rahmstorf 2002

Thermohaline circulation (THC) and meridional overturning circulation (MOC)
Thermohaline circulation is forced not only by differences of temperature and salinity. It is difficult to separate THC from wind driven circulation. Part of its driving energy comes from turbulent mixing crated by wind and tides (as well as zooplankton according to some bold researchers). Wunsch (2002) proposed to use the name THC for transport of sea salt and heat but the part of ocean circulation involving deep waters meridional transport to call Meridional Overturning Circulation (MOC) . This name (and sometimes AMOC where A is for Atlantic) dominates the subject literature of the most recent years. Sir Benjamin Thompson, Rahmstorf (2006) on the other hand defined THC as the “part of the ocean circulation which is driven by fluxes of heat and freshwater across the sea surface and subsequent interior mixing of heat and salt”.
discoverer (1798) of the idea that surface waters transporting heat towards the poles return ad deep cold waters to the tropics.

Is the Gulf Stream a synonym of THC?

SAn early Gulfstream map of 1770 by Beniamin Franklin

“[The] statement that “The Gulf Stream is driven both by the rotation of the Earth and by a deep-water current called the Thermohaline Circulation” is false. The Gulf Stream is a wind-driven phenomenon (as explained in a famous 1948 paper by Henry Stommel). It is part of a current system forced by the torque exerted on the ocean by the wind field. Heating and cooling affect its temperature and other properties, but not its basic existence or structure. As long as the sun heats the Earth and the Earth spins, so that we have winds, there will be a Gulf Stream (and a Kuroshio in the Pacific, an Agulhas in the Indian Ocean, etc).”
Wunsch 2006, Letter to The Economist

Effect of THC stopping on the sea level

Effect of complete stopping of THC on the sea level change (caveat: the scale is not linear!) and change of the surface currents (arrows). The level of North Atlantic and adjacent seas would be about up to 1 meter higher than at present. This is not the effect of ice sheet melting – the change would be almost instantaneous! Levermann et al. 2005 (Climate Dynamics)‫‏‬

Meridional transport of heat: the Atlantic exception

Ocean heat transport (positive means Northward, negative Southward) The Atlantic is the only ocean where thanks to THC, the hear transport crosses the equator and reaches much further North than in the Pacific. With the exception of Indian Ocean monsoon (transporting heat southward), it is the only significant mechanism coupling the climate of Northern and Southern Hemispheres.

Trenberth & Caron, 2001 (Journal of Climate)‫‏‬

Modern view of thermohaline circulation

Deep waters return to the ocean surface thanks to turbulent mixing (especially on ocean ridges) and upwelling driven by Ekman transport around the Antarctic (pushing surface waters north, that is leftwards from the wind direction – Southern Hemisphere!)
Rahmstorf 2006

Observed and reconstructed values of AMO

Using tree-ring width it was possible to reconstruct AMO of past four centuries (caveat: or at least something that well correlates with AMO during the time)
Gray et al. 2004 (Geophysical Research Letters)‫‏‬

Modeled AMO: wavelet analysis

Wavelet analysis of modeled 1400 years of AMO (HadCM3 model) showing a 70-120 year period of AMO.
Knight et al. 2005 (Geophysical Research Letters)‫‏‬

AMO correlates very well with summer Sahel and India rainfall (even better with its low-pass filtered first principal component) and with Atlantic hurricane numbers both for observations (left) and with model (right), for a general circulation model forced with measured Atlantic heat fluxes (CM2.1).

Is it easy to explain the relationships?
Zhang Delworth 2006 (Geophysical Research Letters)‫‏‬

Intertropical Convergence Zone

In the yearly cycle, ITCZ moves always to the hemisphere which receives more heat from the sun see lower panel).

SST changes of North Atlantic [0º to 60º N] (left) and global oceans [60º S to 60º N] (right) show similar temporal variability. Is one of them due to the other?

Subtracting the global trend from North Atlantic SST time series shows the residual AMO. But isn't it strange that it is in phase with global temperature variability?
SST changes of North Atlantic with the global trend subtracted This can be treated as an improved AMO index. Trenberth Shea 2006

(Geophysical Research Letters)‫‏‬

Is AMO another name for global warming? The spatial pattern.

Correlation of the revised AMO index with global surface air temperatures for 1900 to 2004 based on annual values. Only values in the North Atlantic can be considered significant. Is there a good reason for AMO to be in phase wit global temperature? Trenberth Shea 2006 (Geophysical Research Letters)‫‏‬

There is only one problem...
It seems we do not need THC to explain the average changes of Earth temperature. Warming of the 1920s and 1930s was caused by increasing solar activity and green house gases and a break in volcanism. The cooling of 1950s and 1960s was due to more aerosol emission (industrial sulfur rich smoke) and increase of volcanism. The last 30 years were dominated by greenhouse gases increase.
Meehl et al. 2003, 2004 (Journal of Climate)‫‏‬

Brightening of Earth (since 1990)

Most stations measuring aerosol optical thickness note increasing irradiation (less aerosol) since 1990. Only tropical stations in India subcontinent, Africa and South America show decrease of irradiation. Especially the Northern Hemisphere became clearer since the fall of Soviet era heavy industry.

Wild et al. 2005 (Science)‫‏‬

Again: why is AMO in phase with greenhouse and aerosol forced global warming?

Northern Hemisphere temperatures measured and modeled using AMO forced (left) and greenhouse gas forced (right) general circulation model. Both allegedly explain the observed variability. This looks too good to be true. Unless...
Zhang Delworth Held 2007 (Geophysical Research Letters)‫‏‬

A reminder: THC volume depends on the inflow of fresh water to North Atlantic
Model of NADW production (top) and SST (bottom) of present day North Atlantic (left) and glaciation era Atlantic (right) as a function of fresh water inflow flux into Subarctic (red) and Polar (black) Atlantic

Climate models show that fresh water influences NADW production in nonlinear way (with a hysteresis). In order to stop THC, nature needs more fresh water added far from the Arctic (in the Tropical Atlantic) or less added in the Subarctic. In the glacial times the hysteresis loop was narrower (due smaller NADW producing basins) which caused the climate to be unstable.
Ganopolski & Rahmstorf 2001 (Nature)‫‏‬

We have a mechanism for AMO
Unforced modeling with HadCM3 general circulation model shows a salinity pulse followed by freshwater pulse traveling from tropics northwards about circa 100 years. The mechanism is as follows:

Because the anthropogenic forcings (especially aerosol related but also, to a lesser degree greenhouse gases) are stronger in the Northern Hemisphere, they create a inter-hemisphere temperature gradients. This moves ITCZ, influencing THC in similar manner as AMO itself. I believe it possible that we created a forced oscillator AMO pattern, forcing its phase to be in step with the sum anthropogenic forcings. How could we possibly test the hypothesis? Only by using coupled general circulation models.

Another index, highly correlated wit NAO is Arctic Oscillation – difference of air pressure between 37º-45º N zone and the polar vortex (and therefore a measure of its intensity). With positive AO, polar vortex is stronger ans the storm tracks are closer to the pole. Most researchers treat NAO as the local Atlantic chapter of AO.
Ganopolski i Rahmstorf, 2002

Winter NAO index values since 1950

The characteristic feature are low values in the 1960s and high in the 1990s.
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/season.JFM.nao.gif

The number of scientific articles on NAO

Wanner et al. 2001

Can we predict NAO values?
Correlations of summer ocean SST with NAO indexm of the following winter (Rodwell et al. 1999)

British Met Office has a hindcasting 2/3 successful prediction of winter NAO sign from North Atlantic SST of the previous summer.

Rodwell, Rowell & Folland 1999 (Nature) & www.metoffice.gov.uk

NAO leads THC by 10 years!

NAO index (shaded) seems to lead North Atlantic temperature (a simple measure of THC volume) by 10 years (thick line is a 11-year running mean). The mechanism explaining the phenomenon is supposed to be the NAO effect on Labrador Sea deep convection (top is Labrador Sea Water thickness in meters). Latif et al. 2006 (Journal of Climate)‫‏‬

But then, NAO starts in the stratosphere

It seems NAO-like vortex circulation starts in the stratosphere and descends from the 50 km altitude within 60 days to the surface level.
Northern Annular Mode is another name of NAO / AO (compare http://ao.atmos.colostate.edu/introduction.html).

Baldwin & Dunkerton 2001 (Science)‫‏‬

Ozone controls SAM („southern NAO”)?

The trend (1979-2000) of deepening low pressure and the Antarctic vortex is consistent with observed increase of Southern Annual Mode (SAM – the southern counterpart of AO). This deepens the thermal isolation of the Antarctic. Anomalies of geopotential height (an inverted measure of AO) start from stratosphere which temperature is influenced by the presence of ozone. The isolation of the polar vortex deepens the ozone hole. The covariance of stratospheric ozone and SAM seem an established fact. But does it prove Thompson & Solomon 2002 (Science)‫‏‬ causality?

Ozone and NAO

Polar stratospheric ozone for Northern and Southern 63º-90º polar areas (top) and inverted NAO index (bottom) – with the same time scale. Is the covariance accidental? If not, which controls which? The effect can be “explained” qualitatively both ways. But which is true? Or maybe both?

AMO is a free oscillation of THC (but which phase may be controlled by anthropogenic forcings since the 19th century)
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Atlantic SST leads NAO by six months

NAO changes start from the stratosphere with a two month warning before they reach the surface  NAO may be covariant with ozone hole (with uncertain causality) but evidence seems to support NAO as a leading indicator.
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Does it make sense? Certainly not. But this is the great thing about science frontiers!

Thanks for attention

Next:
Climate change threats, Part I: Changes in the climate of the tropic